1. Field of the Invention
The present invention relates to a magnetic recording medium, a method for producing the same, and a magnetic storage apparatus. In particular, the present invention relates to a magnetic recording medium of the type in which a head makes contact therewith temporarily or steadily, such as in the case of a hard disk or a floppy disk. The present invention also relates to a method for producing the magnetic recording medium, and a magnetic storage apparatus.
2. Related Art
In response to the development of the advanced information society in recent years, there is a steady increase in need for the realization of high capacity and high density of the information-recording apparatus. A magnetic storage apparatus is known as one of information-recording apparatuses to respond to the need as described above. The magnetic storage apparatus is used as a large capacity storage unit, for example, for large servers, parallel-connected type computers, personal computers, network servers, movie servers, and mobile PC""s. The magnetic storage apparatus comprises a magnetic recording medium on which information is recorded, and a magnetic head which is used to record and reproduce information on the magnetic recording medium. The magnetic recording medium includes a ferromagnetic thin film composed of cobalt alloy or the like which is formed as a recording layer on a disk-shaped substrate, for example, by means of the sputtering method. A protective film and a lubricant film are formed on the recording layer in order to enhance the resistance to sliding movement and the corrosion resistance.
In association with the realization of the high capacity of the magnetic storage apparatus, the improvement in recording density of the magnetic recording medium is advanced on the basis of the recording of fine and minute recording magnetic domains in the recording layer of the magnetic recording medium. The perpendicular magnetic recording system attracts attention as a method for finely recording the recording magnetic domains. In the perpendicular magnetic recording system, the magnetic recording is performed by forming magnetic domains having perpendicular magnetization in the recording layer by using the magnetic recording medium having the recording layer which exhibits perpendicular magnetization. In the perpendicular magnetic recording system as described above, the minute magnetic domains can be formed in the recording layer. Therefore, it is possible to increase the recording density of the magnetic recording medium.
A polycrystal film based on the Coxe2x80x94Cr system has been hitherto used as a material for the recording layer of the magnetic recording medium in accordance with the perpendicular magnetic recording system as described above. The polycrystal film has a structure in which a Co-rich area having ferromagnetism and a non-magnetic Cr-rich area are separated from each other. The magnetic interaction, which may be exerted between the adjacent ferromagnetic areas, is blocked by the non-magnetic area. Accordingly, the high density and the low noise are realized.
In order to efficiently apply the magnetic field from the magnetic head to the recording layer in the perpendicular magnetic recording system, a magnetic recording medium provided with two layers of magnetic films has been suggested, in which a soft magnetic layer composed of a soft magnetic material and a recording layer composed of a hard magnetic material for recording information are combined.
In order to further improve the areal recording density of the magnetic recording medium, it is necessary to reduce the medium noise. It has been revealed that the development of fine unit of inversion of magnetization (or recorded bits) and the development of high sensitivity of the reading head are effective for this purpose. Especially, it has been revealed that the size of the magnetic crystal grain may be made fine in order to realize the fine unit of inversion of magnetization. However, if the magnetic crystal grain is made too fine, the so-called thermal fluctuation is caused, in which the magnetization state of the magnetic crystal grain is thermally unstable. In order to avoid such an inconvenience, for example, Japanese Laid-Open Patent Publication No. 8-30951 discloses a magnetic recording medium comprising a soft magnetic layer, a first intermediate layer composed of carbon, a second intermediate layer, and a recording film having an artificial lattice structure which are stacked in this order on a non-magnetic substrate.
A magnetic layer, which has magnetic anisotropy higher than that of the polycrystal film based on the Coxe2x80x94Cr system as described above and which is excellent in resistance to the thermal fluctuation, has been progressively studied as a recording layer for the magnetic recording medium. Those known as such a magnetic layer include, for example, an artificial lattice multilayered film (also referred to as xe2x80x9calternately stacked multilayered filmxe2x80x9d) in which Co and Pd or Co and Pt are alternately stacked, and a ordered alloy film which is obtained by thermally treating an alloy film of, for example, Fe and Pt or Co and Pt at a high temperature. The artificial lattice multilayered film and the ordered alloy film are expected to have high resistance to the thermal fluctuation, because such films have high magnetic anisotropy.
However, such a film has the following drawback unlike the polycrystal film based on the Coxe2x80x94Cr system, because the magnetic interaction is strong in the in-plane direction (direction parallel to the surface of the substrate). That is, it is impossible to form small magnetic domains, and the transition medium noise is large. In the case of the magnetic recording medium disclosed in Japanese Laid-Open Patent Publication No. 8-30951 described above, the second intermediate layer composed of Pt or Pd is provided on the first intermediate layer composed of carbon formed on the soft magnetic layer, and the artificial lattice film of Co/Pt or Co/Pd is formed thereon. Accordingly, the crystal orientation of the artificial lattice film is improved, the perpendicular magnetic anisotropy is enhanced, and thus the coercivity is improved. However, in the case of such a magnetic recording medium, the magnetic exchange coupling force in the in-plane direction of the recording layer is strengthened, and the transition noise, which appears as the jitter when the linear recording density is increased, is increased. As a result, it has been difficult to perform recording and reproduction at a high recording density. Further, the following problem also arises. That is, the writing magnetic field supplied from the magnetic head does not arrive at the soft magnetic layer effectively, and the saturation recording characteristics are inferior, because the two intermediate layers, i.e., the first intermediate layer and the second intermediate layer are used.
Japanese Patent No. 2727582 discloses a perpendicularly magnetizable film comprising an artificial lattice film of Co-Pt stacked on an underlying base film composed of composite oxide based on any oxide of Fe, Co, and Ni or an arbitrary combination thereof, as a perpendicular magnetic recording film which is excellent in practical characteristics such as corrosion resistance and durability and which is excellent in perpendicular magnetization characteristics and magneto-optical characteristics.
The present invention has been made in order to solve the problems involved in the conventional technique as described above, an object of which is to provide a magnetic recording medium and a method for producing the same, in which the magnetic exchange coupling force in the in-plane direction of a magnetic layer is low, the transition noise is reduced, and information can be reproduced at high S/N.
Another object of the present invention is to provide a magnetic recording apparatus provided with excellent thermal fluctuation resistance characteristics in which information can be reproduced at high S/N even when the information is recorded at a high areal recording density.
According to a first aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
a soft magnetic layer;
a first seed layer containing oxide of Fe;
a second seed layer containing one of Pd and Pt, Si, and N; and
a recording layer.
The magnetic recording medium according to the first aspect of the present invention comprises, as the underlying base for the recording layer, the second seed layer containing one of Pd and Pt, Si, and N. The magnetic recording medium further comprises, as the underlying base for the second seed layer, the first seed layer containing Fe oxide or oxide of Fe. For example, when the recording layer is constructed with an artificial lattice film composed of a platinum group metal and Co, the first seed layer and the second seed layer as described above make it possible to optimally control the crystalline orientation of the artificial lattice film and the magnetic exchange coupling force of crystal grains.
According to studies performed by the present inventors, when the second seed layer was formed of, for example, only Pd crystals, then the size of the recording magnetic domain formed in the recording layer was increased, and it was impossible to form any fine and minute recording magnetic domain, probably for the following reason. That is, if the second seed layer is formed of only Pd crystals, then a recording layer having an artificial lattice structure, in which the grain boundary is indistinct, is formed on the second seed layer, and the magnetic exchange coupling force in the in-plane direction, which is exerted between crystal grains of the recording layer, is strengthened. The present inventors have found out that it is possible to form fine and minute magnetic domains in the recording layer, and it is possible to reduce the noise as well, by constructing the second seed layer with one of Pd and Pt, Si, and N, probably for the following reason.
It is considered that when the second seed layer is constructed with one of Pd and Pt, Si, and N, Pd or Pt exists in a dispersed manner as a microcrystalline or partially amorphous structure in SiN (or in SiN network structure). Further, the dispersion of Pd or Pt in SiN in the second seed layer as described above is facilitated in an advanced manner in accordance with the principle as described later on, owing to the fact that the first seed layer containing Fe oxide is used as the underlying base. It is considered that the artificial lattice film having the distinct grain boundary is formed on the second seed layer, because the recording layer, which has the artificial lattice structure formed on the second seed layer, is grown with nucleuses of dispersed Pd or Pt. Therefore, the magnetic exchange coupling force in the in-plane direction, which is exerted between the crystal grains of the recording layer having the artificial lattice structure, is reduced, and thus the transition noise is reduced. Especially, the trace amount of N in the second seed layer can further accelerate the dispersion of Pd or Pt as a result of binding to Si. Therefore, it is possible to further weaken the magnetic exchange coupling force in the in-plane direction of the recording layer. Accordingly, it is possible to further reduce the transition noise.
The reason why the dispersion of Pd or Pt in SiN in the second seed layer is further facilitated by using the first seed layer containing the Fe oxide as the underlying base for the second seed layer will now be explained.
According to the knowledge of the present inventors, Pd or Pt, which is the metal element of the elements for constructing the second seed layer, has low wettability with respect to the Fe oxide for constructing the first seed layer. For this reason, it is considered that when the second seed layer containing Pd or Pt is formed on the first seed layer containing the Fe oxide, Pd or Pt, which has the low wettability with respect to the Fe oxide, is formed in a further dispersed manner on the layer of the Fe oxide in accordance with the surface tension. Accordingly, it is considered that the dispersion is further accelerated for Pd or Pt existing in the microcrystalline or partially amorphous structure in SiN (or SiN network structure). As described above, when the second seed layer, which contains one of Pd and Pt, Si, and N, is formed on the first seed layer containing the Fe oxide, and the recording layer is formed on the second seed layer, then the aggregates of extremely fine crystal grains are formed in the recording layer in accordance with the principle described above. It is possible to form minute magnetic domains in the recording layer constructed by the aggregates of fine crystal grains. Further, the magnetization transition area is extremely distinct as well. Therefore, it is possible to reduce the noise as compared with the conventional technique.
In the magnetic recording medium according to the first aspect of the present invention, the contents of Si and N in the second seed layer are desirably as follows. That is, the content of Si is desirably within a range of 10 atomic % to 35 atomic %, and more desirably 20 atomic % to 30 atomic %. The content of N is desirably within a range of 0.1 atomic % to 5 atomic %, and more desirably 0.5 atomic % to 5 atomic %. When the contents of Si and N in the second seed layer are controlled to be within the ranges as described above, it is possible to optimize the crystalline orientation of the recording layer and the magnetic exchange coupling force in the in-plane direction. Accordingly, it is possible to reliably form the fine and minute recording magnetic domains in the recording layer, and the magnetization transition area is distinct as well. Thus, it is possible to reduce the noise. That is, it is possible to realize the reduction of the noise and the improvement of the resolution. The second seed layer may further contain a trace amount of Co. In this case, it is preferable that the content of Co is within a range of 1 atomic % to 10 atomic %, while the contents of Si and N in the second seed layer satisfy the ranges described above. It is preferable that the second seed layer has a microcrystalline structure or a structure in which amorphous matters partially exist in a microcrystalline structure.
In the magnetic recording medium according to the first aspect of the present invention, it is preferable that the first seed layer contains Fe existing as metal (hereinafter referred to as xe2x80x9cFe metalxe2x80x9d) in addition to the Fe oxide. The magnetic recording medium provided with the seed layer as described above makes it possible to further reduce the medium noise. The reason therefor will be explained below.
The first seed layer, which contains the Fe metal in addition to the Fe oxide, is considered to be in a state in which extremely minute Fe metal particles are dispersed in the Fe oxide. As described above, the Fe oxide has the low wettability with respect to Pd or Pt, for example, for constructing the second seed layer. On the other hand, the Fe metal has the high wettability with respect to Pd or Pt. For this reason, when Pd or Pt is accumulated on the first seed layer in which the Fe metal particles are dispersed in the Fe oxide, Pd or Pt selectively adsorbs to the Fe metal. In this situation, the Fe metal in the first seed layer is extremely minute. Therefore, Pd or Pt, which has adsorbed to the Fe metal, is more minute as compared with a case in which Pd or Pt is formed on the seed layer composed of the Fe oxide described above. Further, the Fe oxide, which has the low wettability with respect to Pd or Pt, exists around the Fe metal. Therefore, Pd or Pt, which has been accumulated on the first seed layer, is restricted for the spread two-dimensionally, i.e., in the in-plane direction. Pd or Pt is individually dispersed at predetermined spacing distances while maintaining the minute state. Therefore, it is considered that Pd or Pt in SiN (or SiN network structure) of the second seed layer exists in a dispersed manner in an extremely minute state. When the recording layer is formed on the second seed layer as described above, the magnetic grains of the recording layer are grown in units of finely dispersed Pd or Pt. Therefore, the recording layer is obtained, which is formed of the fine and minute magnetic grains. Accordingly, the magnetic domains, which are formed in the recording layer, are also fine and minute. Thus, it is possible to further reduce the noise.
In the present invention, it is preferable that a ratio between numbers of atoms (FeMet/FeOxi) satisfies a relationship of 0.02 less than (FeMet/FeOxi)  less than 0.2 provided that FeMet represents the number of atoms of Fe existing as metal in the first seed layer, and FeOxi represents the number of atoms of Fe existing as oxide. When the ratio between numbers of atoms is larger than 0.02, then it is possible to record information at a high density in the recording layer, and it is possible to reproduce the information at high S/N. However, if the ratio between numbers of atoms is larger than 0.2, it is feared that the Fe metal exists in an excessive amount in the seed layer, the selectivity disappears for the adsorption of the platinum group element, and it is impossible to form fine magnetic grains in the recording layer.
In the magnetic recording medium according to the first aspect of the present invention, it is preferable that the first seed layer containing the Fe oxide contains the Fe oxide in an amount of not less than 80% by volume as a whole.
In the magnetic recording medium according to the first aspect of the present invention, it is desirable that both of the film thicknesses of the first and second seed layers are within a range of 1 nm to 30 nm. If both of the film thicknesses of the first and second seed layers are less than 1 nm, it is feared that the crystalline orientation of the recording layer having the artificial lattice structure on the seed layer cannot be controlled. If both of the film thicknesses of the first and second seed layers are thicker than 30 nm, it is feared that the distance between the soft magnetic layer and the magnetic pole of the recording magnetic head is increased, and the recording magnetic field is not sufficiently applied from the recording magnetic head to the recording layer. Further, it is feared that the magnetic field from the recording magnetic head is applied to the recording layer in a state in which the magnetic field is widened, resulting in the decrease in resolution and the increase in disturbance of the magnetization transition area to cause any noise based on the jitter.
According to a second aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
a soft magnetic layer;
a seed layer containing one of Pd and Pt, Si, and N; and
a recording layer.
The magnetic recording medium of the present invention comprises, as an underlying base for the recording layer, the seed layer containing one of Pd and Pt, Si, and N. Such a seed layer has the same function as that of the second seed layer according to the first aspect of the present invention. Therefore, it is possible to optimally control the magnetic exchange coupling force between crystal grains and the crystalline orientation of the artificial lattice structure formed on the seed layer.
That is, it is considered that Pd or Pt exists in a dispersed manner in a microcrystalline or partially amorphous structure in SiN (or SiN network structure), when the seed layer is constructed with one of Pd and Pt, Si, and N. Further, it is considered that the artificial lattice film having the distinct grain boundary is formed, because the recording layer, which has the artificial lattice structure grown on the seed layer, is grown with nucleuses of dispersed Pd or Pt. Therefore, the magnetic exchange coupling force in the in-plane direction, which is exerted between the crystal grains of the recording layer having the artificial lattice structure, is reduced. Especially, the trace amount of N in the seed layer is bound to Si, and thus it is possible to further facilitate the dispersion of Pd or Pt. Therefore, it is possible to further weaken the magnetic exchange coupling force in the in-plane direction of the recording layer. Accordingly, it is possible to further reduce the transition noise.
In the magnetic recording medium according to the second aspect of the present invention, the contents of Si and N in the seed layer are desirably as follows. That is, the content of Si is desirably within a range of 10 atomic % to 35 atomic %, and more desirably 20 atomic % to 30 atomic %. The content of N is desirably within a range of 0.1 atomic % to 5 atomic %, and more desirably 0.5 atomic % to 5 atomic %. When the contents of Si and N in the seed layer are controlled to be within the ranges as described above, it is possible to optimize the crystalline orientation of the recording layer and the magnetic exchange coupling force in the in-plane direction. Accordingly, it is possible to reliably form the fine and minute recording magnetic domains in the recording layer, and the magnetization transition area is distinct as well. Thus, it is possible to reduce the noise. That is, it is possible to realize the reduction of the noise and the improvement of the resolution. The seed layer may further contain a trace amount of Co. In this case, it is preferable that the content of Co is within a range of 1 atomic % to 10 atomic %, while the contents of Si and N in the seed layer satisfy the ranges described above. It is preferable that the seed layer has a microcrystalline structure or a structure in which amorphous matters partially exist in a microcrystalline structure.
In the magnetic recording medium according to the second aspect of the present invention, it is desirable that the film thickness of the seed layer is within a range of 1 nm to 30 nm. If the film thickness of the seed layer is less than 1 nm, it is feared that it is impossible to control the crystalline orientation of the recording layer having the artificial lattice structure thereon. If the film thickness of the seed layer is thicker than 30 nm, it is feared that the distance between the soft magnetic layer and the magnetic pole of the recording magnetic head is increased, and the recording magnetic field is not sufficiently applied from the recording magnetic head to the recording layer. Further, it is feared that the magnetic field from the recording magnetic head is applied to the recording layer in a state in which the magnetic field is widened, resulting in the decrease in resolution and the increase in disturbance of the magnetization transition area to cause any noise based on the jitter.
In the magnetic recording media according to the first and second aspects of the present invention, the recording layer may be a recording layer having an artificial lattice structure. Preferably, the recording layer having such an artificial lattice structure is principally composed of a platinum group metal and Co, and it is preferably an alternately stacked multilayered film in which the platinum group element and Co are alternately stacked substantially in a thickness of several atoms or substantially in a thickness of single atom. For example, at least one of Pt and Pd may be used for the platinum group element. The alternately stacked multilayered film as described above can be formed as a film at the room temperature or at a relatively low substrate temperature. Further, the alternately stacked multilayered film has large magnetic anisotropy, and hence it is most preferred to be used for the recording layer for the high density recording.
In this specification, the term xe2x80x9cartificial lattice structurexe2x80x9d means a structure which is obtained such that a plurality of different substances are stacked mutually periodically in a certain direction in a thickness of single atom or in a thickness of several atoms. The film, which has the artificial lattice structure as described above, is also referred to as xe2x80x9cartificial lattice filmxe2x80x9d or xe2x80x9calternately stacked multilayered filmxe2x80x9d.
The recording layer having the artificial lattice structure is desirably a Co/Pd artificial lattice film formed by alternately stacking a Co layer which has a film thickness selected from those within a range of 0.05 nm to 0.5 nm, and a Pd layer which has a film thickness selected from those within a range of 0.5 to 2 nm, or a Co/Pt artificial lattice film formed by alternately stacking a Co layer which has a film thickness selected from those within a range of 0.05 nm to 0.5 nm, and a Pt layer which has a film thickness selected from those within a range of 0.1 to 2 nm. The perpendicular magnetic anisotropy is expressed most readily in the artificial lattice film having the structure as described above.
In the magnetic recording media according to the first and second aspects of the present invention, when the recording layer is formed by using the Co/Pd artificial lattice film or the Co/Pt artificial lattice film as described above, an additive element may be contained in the Pd layer or the Pt layer. The fluctuation of the composition occurs when the additive element is contained in the Pd layer or the Pt layer as described above. Thus, it is possible to reduce the magnetic exchange coupling force in the in-plane direction of the recording layer. The additive element is desirably Si, Al, Zr, Ti, or B, and especially preferably B. The magnetic characteristics are less deteriorated when the additive element is added to the Pd layer or the Pt layer, as compared with a case in which the additive element is added to the Co layer.
It is preferable that Co in the Co/Pd artificial lattice film or the Co/Pt artificial lattice film is distributed discontinuously in the in-plane direction. The phrase xe2x80x9cCo in the artificial lattice film is distributed discontinuously in the in-plane directionxe2x80x9d herein means the provision of the following structure. That is, when the cross-sectional structure of the artificial lattice film is observed, the cross section of the Co layer is observed to be substantially layered, while when the planar structure is observed, areas composed of Co are dispersed like islands on the plane. In other words, the Co layer in the artificial lattice film is not formed as a continuous film, but a plurality of areas composed of Co are dispersed like islands. Co, which is distributed discontinuously in the artificial lattice film, partially blocks the magnetic exchange coupling force. Therefore, it is possible to reduce the magnetic exchange coupling force in the in-plane direction of the recording layer.
The recording layer having the artificial lattice structure may be formed, for example, with aggregates of columnar (column-shaped) crystal grains. The diameter in a cross section perpendicular to the rotation axis of the columnar crystal grain may be within a range of 2 nm to 15 nm. The difference between the uppermost portion and the lowermost portion (height position of the grain boundary of the crystal grain) disposed on the surface of the crystal grain may be within a range of 1 nm to 10 nm. In the recording layer having the structure as described above, the magnetic exchange coupling force in the in-plane direction is reduced. Even when minute recording magnetic domains are formed in the recording layer, the magnetic domains exist in a stable manner. Further, the linearity of the magnetization transition area is high as well. Therefore, it is possible to further reduce the noise upon the reproduction.
In the magnetic recording media according to the first and second aspects of the present invention, the recording layer having the artificial lattice structure can be formed, for example, by using a sputtering apparatus which is capable of forming films alternately. For example, the recording layer can be formed as follows. That is, two or more targets, which are composed of different materials, are juxtaposed, and a substrate carrier is relatively moved alternately with respect to the respective targets. Alternatively, the recording layer can be formed as follows. That is, at least two types of ring-shaped targets having different diameters are arranged coaxially on an identical plane. A substrate is arranged so that the substrate is opposed to the targets. The film is formed by alternately effecting the discharge with the ring-shaped targets.
The film thickness of the recording film having the artificial lattice structure is preferably 5 nm to 60 nm in view of the magnetic characteristics. It is desirable for the recording layer that the coercivity, which is measured in the direction perpendicular to the substrate surface, is 1.5 kOe to 10 kOe (kilooersted). It is desirable that (Mrxc2x7t), which is the product of the film thickness t of the recording layer and the residual magnetization Mr, is within a range of 0.3 to 1.0 memu/cm2. If the coercivity is smaller than 1.5 kOe, it is feared that the output, which is obtained when information recorded at a high density (not less than 600 kFCI) is reproduced, is small. Further, it is feared that the magnetic anisotropy energy is decreased, and the thermal fluctuation tends to occur. If the value of Mrxc2x7t is larger than 1.0 memu/cm2, the resolution is lowered. If the value of Mrxc2x7t is smaller than 0.3 memu/cm2, the output is excessively small. Therefore, it is feared that it is difficult to obtain sufficient recording and reproducing characteristics when the high density recording is performed at not less than 150 gigabits/square inch.
According to a third aspect of the present invention, there is provided a magnetic recording medium comprising:
a soft magnetic layer;
a recording layer which is formed with a hard magnetic material and which exhibits perpendicular magnetization; and
a seed layer which is located between the soft magnetic layer and the recording layer and which contains oxide of Fe.
The magnetic recording medium according to the third aspect of the present invention comprises the recording layer which is formed with the hard magnetic material to exhibit the perpendicular magnetization for recording information thereon, and the seed layer which contains the Fe oxide disposed between the recording layer and the soft magnetic layer formed with the soft magnetic material. Nucleuses of magnetic grains for constructing the recording layer can be grown at predetermined spacing distances on the surface of the seed layer. It is preferable that the recording layer contains a platinum group element. It is especially preferable that the recording layer is an alternatively stacked multilayered film obtained by alternately stacking a platinum group element and Co element. The recording layer, which is composed of aggregates of fine magnetic grains, can be formed by forming the seed layer containing the Fe oxide as an underlying base for the recording layer. The reason therefor will be explained below.
The Fe oxide, which is contained in the seed layer, has low wettability with respect to the platinum group element such as Pt and Pd for constructing the recording layer. For this reason, when Pt or Pd is accumulated on the seed layer as described above, for example, by using the sputtering method, Pt or Pd is formed while being finely dispersed in the in-plane direction on the seed layer in accordance with the surface tension. Pt or Pd, which exists in the finely dispersed manner on the seed layer as described above, serves as the nucleus for growing the magnetic grains of the recording layer. Therefore, when Co and Pt or Pd are alternately accumulated thereon, the magnetic grains are grown from the nucleuses individually in an isolated state. The magnetic grains grown as described above use the finely dispersed nucleuses as the units, and hence the magnetic grains, which are relatively fine and minute, are obtained on the seed layer. The recording layer is formed by the aggregates of such fine magnetic grains. Therefore, it is possible to form minute recording magnetic domains, and the magnetic interaction between the magnetic grains of the recording layer is reduced as well. Further, the boundary portion between the recording magnetic domains is distinct. Therefore, it is possible to reduce the noise.
In the magnetic recording medium according to the third aspect of the present invention, it is preferable that the seed layer contains Fe existing as metal (Fe metal) in addition to the Fe oxide. The magnetic recording medium, which is provided with the seed layer as described above, makes it possible to further reduce the medium noise. The reason therefor will be explained below.
It is considered that the seed layer, which contains the Fe metal in addition to the Fe oxide, is in a state in which extremely minute Fe metal particles are dispersed in the Fe oxide. As described above, the Fe oxide has the low wettability with respect to the platinum group element such as Pd or Pt as the element, for example, for constructing the recording layer. On the other hand, the Fe metal has the high wettability with respect to Pd or Pt. For this reason, when Pd or Pt is accumulated on the seed layer in which the Fe metal particles exist in the dispersed manner in the Fe oxide, Pd or Pt selectively adsorbs to the Fe metal. In this situation, the Fe metal in the seed layer is extremely minute. Therefore, Pd or Pt, which has adsorbed to the Fe metal, is more minute as compared with a case in which Pd or Pt is formed on the seed layer composed of the Fe oxide described above. Further, the Fe oxide, which has the low wettability with respect to Pd or Pt, exists around the Fe metal. Therefore, Pd or Pt, which has been accumulated on the seed layer, is restricted for the spread two-dimensionally, i.e., in the in-plane direction. Pd or Pt is individually dispersed at predetermined spacing distances while maintaining the minute state. As described above, Pd or Pt, which is dispersed extremely minutely, serves as the nucleus for growing the magnetic grains of the recording layer. Therefore, when Co and Pd or Pt are alternately accumulated thereon, the magnetic grains of the recording layer are grown from the minute nucleuses. That is, when the seed layer, which contains the Fe oxide and the Fe metal, is used as the underlying base for the recording layer, the Fe metal in the seed layer plays a role of the nucleus for growing the extremely fine magnetic grains in the recording layer. Further, the magnetic grains are grown by using the minute nucleuses as the units. Therefore, the recording layer, which is formed with the minute magnetic grains, is obtained. Accordingly, the magnetic domains, which are formed in the recording layer, are also fine and minute. Thus, it is possible to further reduce the noise.
In the magnetic recording medium according to the third aspect of the present invention, it is preferable that a ratio between numbers of atoms (FeMet/FeOxi) satisfies 0.02 less than (FeMet/FeOxi) less than 0.2 provided that FeMet represents the number of atoms of Fe existing as metal in the seed layer, and FeOxi represents the number of atoms of Fe existing as oxide. As illustrated by embodiments described later on, when the ratio between numbers of atoms is larger than 0.02, then it is possible to record information at a high density on the recording layer, and it is possible to reproduce the information at high S/N. However, if the ratio between numbers of atoms is larger than 0.2, it is feared that the Fe metal exists in an excessive amount in the seed layer, the selectivity disappears for the adsorption of the platinum group element, and it is impossible to form fine magnetic grains in the recording layer.
In the magnetic recording medium according to the third aspect of the present invention, it is preferable that the seed layer, which contains the Fe oxide, contains the Fe oxide in an amount of not less than 80% by volume as a whole. When the seed layer is formed by oxidizing the soft magnetic layer at a high temperature as described later on, an impurity may be contained by about 10 atomic % in addition to the Fe oxide or the Fe metal.
In the magnetic recording medium according to the third aspect of the present invention, it is preferable that the film thickness of the seed layer is not more than 30 nm in order not to lower the recording efficiency due to the magnetic spacing.
In the magnetic recording medium according to the third aspect of the present invention, the recording layer, which is formed with the hard magnetic material, may be a perpendicularly magnetizable film having magnetization in the vertical direction with respect to the film surface. A ordered alloy film can be used for the recording layer as described above in addition to the artificial lattice multilayered film (alternately stacked multilayered film) used in the first and second aspects. The hard magnetic material is preferably a material principally composed of a platinum group element and Co. At least one element of Pt and Pd is preferred for the platinum group element. It is preferable to form the recording layer by using the alternately stacked multilayered film obtained by alternately stacking the platinum group element and Co. The alternately stacked multilayered film and the ordered alloy film are excellent in productivity, because they can be formed as a film at the room temperature or at a relatively low substrate temperature. Further, the alternately stacked multilayered film and the ordered alloy film are excellent in thermal fluctuation resistance characteristics, because they have high magnetic anisotropy. Therefore, the alternately stacked multilayered film and the ordered alloy film are extremely optimum to be used as the recording layer for the high density recording.
In the magnetic recording media according to the first to third aspects of the present invention, the soft magnetic layer is preferably composed of a soft magnetic film having a microcrystalline structure obtained by uniformly dispersing, in Fe, nitride or carbide of at least one element selected from Ta, Nb, and Zr, in view of the fact that the magnetic field from the magnetic head is efficiently applied to the recording layer. Other than the materials as described above, for example, an amorphous alloy may be used, which is principally composed of Coxe2x80x94Zr and which contains at least one element selected from Ta, Nb, and Ti. The soft magnetic film as described above is suitable for high density recording, because the film has a large saturation magnetic flux density of not less than 1.5 T. Those usable as specified materials include, for example, NiFe, CoTaZr, CoNbZr, and FeTaC having a high magnetic permeability. The magnetic layer composed of such a material can be formed, for example, in accordance with the sputtering method and the vapor deposition method at a film thickness of not more than 1000 nm.
In the magnetic recording medium according to the second aspect of the present invention, it is preferable that the surface of the soft magnetic layer is flat. It is preferable that the surface roughness Ra of the surface of the soft magnetic layer is 0.20 nm to 0.40 nm. When the soft magnetic layer having the flat surface as described above is used, the boundary between the magnetic crystal grains of the recording layer, i.e., the crystal grain boundary is extremely distinct as illustrated in embodiments described later on. Thus, the isolation of the magnetic crystal grains of the recording layer is further facilitated. The magnetic crystal grains of the recording layer as described above are magnetically separated from each other by the crystal grain boundary. Therefore, the magnetic exchange coupling force in the in-plane direction is reduced. Accordingly, it is possible to form minute magnetic domains in the recording layer, and the linearity of the magnetization transition area is enhanced. The fact that the crystal grain boundary of the recording layer is distinct owing to the flat surface of the soft magnetic layer is considered to be based on the following principle.
It is considered that when the seed layer is formed as a film on the soft magnetic layer, if any irregularity exists on the surface of the soft magnetic layer, then sputtering particles are captured by the irregularity. For this reason, it is considered that an initial growth layer, in which grains for constructing the seed layer are grown without being separated from each other by sufficient spacing distances, is formed on the soft magnetic layer. On the contrary, when the surface of the soft magnetic layer is flat, sputtering particles, which have arrived at the surface of the soft magnetic layer, are sufficiently diffused in the in-plane direction. Therefore, an initial growth layer is formed as a film, in which grains for constructing the seed layer are grown in a state of being sufficiently separated from each other. The seed layer, which is formed on the basis of the initial growth layer with the separation of the sufficient spacing distance, is considered as follows. That is, Pd or Pt, which exists in a microcrystalline or partially amorphous structure in SiN (or SiN network structure), is also separated from each other by sufficient spacing distances, and the dispersion is further facilitated. It is considered that when the recording layer is formed as a film on the seed layer in which the dispersion of Pd or Pt is facilitated in an advanced manner, the extremely distinct crystal grain boundary is obtained in the recording layer. In order to obtain the flat surface of the soft magnetic layer, for example, the surface may be subjected to dry etching after forming the film of the soft magnetic layer.
In the magnetic recording media according to the first to third aspects of the present invention, those usable as the substrate include, for example, non-magnetic substrates such as aluminum-magnesium alloy substrates, glass substrates, and graphite substrates. The surface of the aluminum-magnesium alloy substrate may be plated with nickel-phosphorus. The substrate surface may be treated to be flat by applying diamond grinding grains or polishing tape to the substrate surface while rotating the substrate. Accordingly, when the magnetic head is allowed to float over the magnetic recording medium, it is possible to improve the traveling characteristics of the magnetic head. As for the center line roughness Ra of the substrate surface, it is desirable that the center line roughness of a protective film to be formed on the substrate is not more than 1 nm. In the case of the glass substrate, the surface may be chemically etched with a chemical agent such as strong acid so that the surface is flat. Further, any minute height structure, for example, projections of not more than 1 nm may be chemically formed on the surface. Accordingly, it is possible to realize a stable low floating amount when a negative pressure slider is used.
An adhesive layer such as those of Ti may be formed on the substrate of the magnetic recording medium in order to improve the tight contact performance before forming the film of the soft magnetic layer.
Each of the magnetic recording media according to the first to third aspects of the present invention may comprise a protective layer on the recording layer. Those preferably usable for the protective layer include, for example, any one of amorphous carbon, silicon-containing amorphous carbon, nitrogen-containing amorphous carbon, boron-containing amorphous carbon, silicon oxide, zirconium oxide, and cubic crystal system boron nitride. The method for forming the protective film of such amorphous carbon includes, for example, a method in which the film is formed by means of sputtering in an inert gas or a mixed gas of inert gas and hydrocarbon gas such as methane by using graphite as a target, a method in which the film is formed by means of plasma CVD by using an organic compound such as hydrocarbon gas, alcohol, acetone, and adamantane singly or mixed with, for example, hydrogen gas or inert gas, and a method in which the film is formed by ionizing an organic compound to effect acceleration by applying a voltage in order to make collision with the substrate. Further, the protective film may be formed by means of the ablation method in which a laser beam at a high output is collected with a lens so that the laser beam is radiated onto a target such as graphite.
A lubricant can be applied onto the protective film in order to obtain good characteristics of sliding movement resistance. Perfluoropolyether-based high molecular weight lubricant, which has a principal chain structure composed of three elements of carbon, fluorine, and oxygen, is used as the lubricant. Alternatively, a fluorine-substituted alkyl compound can be also used as the lubricant. Other organic lubricants and inorganic lubricants may be used provided that they are materials which provide stable sliding movement and durability.
The solution application method is generally used as the method for forming the lubricant. In order to avoid the global warming or simplify the process steps, a lubricant film may be formed in accordance with the photo-CVD method in which no solvent is used. The photo-CVD method is performed by radiating ultraviolet light onto a gaseous material composed of olefin fluoride and oxygen.
The film thickness of the lubricant is appropriately 0.5 nm to 3 nm as an average value. If the film thickness is thinner than 0.5 nm, the lubricant characteristics are deteriorated. If the film thickness is thicker than 3 nm, then the meniscus force is increased, and the static frictional force (stiction) between the magnetic head and the magnetic disk is increased, which is not preferred. After the lubricant film is formed as described above, the heat may be applied at about 100xc2x0 C. for 1 to 2 hours in nitrogen or in air. Accordingly, any excessive solvent and low molecular weight components can be evaporated to improve the tight contact performance between the lubricant film and the protective film. Other than the post-treatment as described above, for example, a method may be used, in which ultraviolet light is radiated for a short period of time with an ultraviolet lamp after forming the lubricant film. The same or equivalent effect is also obtained in accordance with such a method.
According to a fourth aspect of the present invention, there is provided a method for producing a magnetic recording medium, comprising:
preparing a substrate;
forming a soft magnetic layer on the substrate;
forming, on the soft magnetic layer, a first seed layer containing oxide of Fe;
forming, on the first seed layer, a second seed layer containing one of Pd and Pt, Si, and N; and
forming, on the second seed layer, a recording layer.
In the production method according to the fourth aspect of the present invention, the first seed layer can be formed, for example, by allowing a target principally containing Fe to be subjected to reactive sputtering by using a sputtering gas containing oxygen. The first seed layer, which is formed in accordance with the method as described above, contains the Fe oxide or the oxide of Fe. According to the production method as described above, it is possible to produce the magnetic recording medium according to the first aspect of the present invention.
In the production method according to the fourth aspect of the present invention, in order to form the second seed layer containing one of Pd and Pt, Si, and N on the first seed layer, for example, the sputtering may be performed by using two types of targets of Pd target and SiN target.
In the fourth production method of the present invention, for example, an artificial lattice film composed of a platinum group element and Co element can be formed as the recording layer having an artificial lattice structure on the second seed layer such that a target formed with the platinum group element and a target formed with Co are used to perform the sputtering while alternately opening/closing shutters for the targets.
In the production method according to the fourth aspect of the present invention, the seed layer, which contains the Fe metal in addition to the Fe oxide, can be formed by controlling the flow rate of the oxygen gas in the sputtering gas. When the seed layer, which contains the Fe oxide and the Fe metal, is used as the underlying base for the recording layer as described above, it is possible to form the aggregates of minute magnetic grains in the recording layer. Therefore, the magnetic recording medium, which is provided with the seed layer as described above, makes it possible to further reduce the medium noise.
According to a fifth aspect of the present invention, there is provided a method for producing a magnetic recording medium, comprising:
preparing a substrate;
forming a soft magnetic layer on the substrate;
forming, on the soft magnetic layer, a seed layer containing one of Pd and Pt, Si, and N; and
forming, on the seed layer, a recording layer.
According to the production method as described above, it is possible to produce the magnetic recording medium according to the second aspect of the present invention.
In the production method according to the fifth aspect of the present invention, it is preferable that the surface of the soft magnetic layer is subjected to an etching treatment, for example, by means of plasma etching after forming the film of the soft magnetic layer on the substrate. Accordingly, it is possible to obtain the soft magnetic layer having a flat surface. When the seed layer, which contains one of Pd and Pt, Si, and N, is formed on the soft magnetic layer having the flat surface, and the recording layer is formed on the seed layer as described above, then the crystal grain boundary of the recording layer is extremely distinct, and the isolation of crystal grains is facilitated. In the magnetic recording medium produced by the production method of the present invention, the magnetic exchange coupling force in the in-plane direction of the recording layer is further reduced. Therefore, the linearity of the magnetization transition area is enhanced, and it is possible to reduce the noise.
According to a sixth aspect of the present invention, there is provided a method for producing a magnetic recording medium, comprising:
preparing a substrate;
forming a soft magnetic layer on the substrate;
forming, on the soft magnetic layer, a seed layer containing oxide of Fe; and
forming a recording layer on the seed layer.
In the production method according to the sixth aspect of the present invention, for example, the seed layer can be formed by allowing a target principally containing Fe to be subjected to reactive sputtering by using a sputtering gas containing oxygen. The seed layer, which is formed in accordance with the method as described above, contains the Fe oxide. According to the production method as described above, it is possible to produce the magnetic recording medium according to the third aspect of the present invention.
In the production method according to the sixth aspect of the present invention, the seed layer, which contains the Fe metal in addition to the Fe oxide, can be formed by controlling the flow rate of the oxygen gas in the sputtering gas. When the seed layer, which contains the Fe oxide and the Fe metal, is used as the underlying base for the recording layer as described above, it is possible to form the aggregates of minute magnetic grains in the recording layer. Therefore, the magnetic recording medium, which is provided with the seed layer as described above, makes it possible to further reduce the medium noise. When the Fe metal is formed in the seed layer by controlling the flow rate of the oxygen gas as described above, it is desirable that the surface of the seed layer is subjected to sputtering etching during the period from the formation of the seed layer to the formation of the recording layer on the seed layer. The reason therefor will be explained below.
The oxygen gas remains in a film-forming chamber after the seed layer is formed by means of the reactive sputtering by using the sputtering gas containing the oxygen gas. A certain period of time is required to completely evacuate the oxygen gas. Therefore, during this period, a thin oxide film is formed on the surface of the Fe metal in the seed layer due to the oxygen gas remaining in the chamber. Such an oxide film inhibits the adsorption of the platinum group element in the recording layer to the Fe metal, for example, when the recording layer containing the platinum group element is formed on the seed layer. Therefore, it is feared that the adsorption selectivity for the platinum group element is lowered. Accordingly, as described above, the oxide film, which is formed on the surface of the Fe metal, is removed by sputtering-etching the surface of the seed layer after forming the film of the seed layer. Thus, the platinum group element such as Pd and Pt for constructing the recording layer can be reliably adsorbed to the Fe metal on the surface of the seed layer. Therefore, it is possible to form the minute magnetic grains in the recording layer. Those preferably used as the gas to be used for the sputtering etching include an inert gas such as Ar, Kr, and Xe, and a mixed gas of such an inert gas and hydrogen gas.
In the production method according to the sixth aspect of the present invention, for example, when a soft magnetic layer containing Fe is used as the soft magnetic layer, the seed layer containing the Fe oxide can be also formed by oxidizing the surface of the soft magnetic layer at a high temperature after forming the soft magnetic layer containing Fe.
In the production methods according to the fourth to sixth aspects of the present invention, those usable as the method for forming the soft magnetic layer, the first seed layer, the second seed layer, and the recording layer include, for example, the vacuum vapor deposition method, the MBE method, the sputtering method, the ion beam method, the molecular layer epitaxy method, and the plasma CVD. Those useable as the sputtering method include, for example, known sputtering methods such as the ECR sputtering method, the DC sputtering method, and the RF sputtering method.
According to a seventh aspect of the present invention, there is provided a magnetic storage apparatus comprising:
the magnetic recording medium according to any one of the first to third aspects;
a magnetic head which is used to record or reproduce information; and
a driving unit which drives the magnetic recording medium with respect to the magnetic head.
The magnetic storage apparatus of the present invention is provided with the magnetic recording medium according to any one of the first to third aspects of the present invention. Therefore, even when information is recorded at a high areal recording density, the information can be reproduced at high S/N. Further, the magnetic storage apparatus possesses excellent thermal fluctuation resistance.
In the magnetic storage apparatus of the present invention, the magnetic head may comprise a recording magnetic head for recording information on the magnetic recording medium, and a reproducing magnetic head for reproducing information recorded on the magnetic recording medium. It is desirable that the gap length of the recording magnetic head is 0.2 xcexcm to 0.02 xcexcm. If the gap length exceeds 0.2 xcexcm, it is difficult to perform recording at a high linear recording density of not less than 400 kFCI. It is difficult to produce a recording head having a gap length smaller than 0.02 xcexcm. In the case of such a recording head, the device tends to be destroyed due to the static electricity induction.
The reproducing magnetic head can be constructed by using a magnetoresistance effect element. It is desirable that the reproduction shield interval of the reproducing magnetic head is 0.2 xcexcm to 0.02 xcexcm. The reproduction shield interval directly relates to the reproducing resolution. The shorter the reproduction shield interval is, the higher the resolution is. It is desirable that the lower limit value of the reproduction shield interval is appropriately selected within the range described above, depending on, for example, the stability of the element, the reliability, the electric resistance characteristics, and the output.
In the magnetic storage apparatus of the present invention, the driving unit can be constructed with a spindle for rotating and driving the magnetic recording medium. It is desirable that the velocity of rotation of the spindle is 3000 revolutions per minute to 20000 revolutions per minute. If the velocity of rotation of the spindle is slower than 3000 revolutions per minute, the data transfer speed is low, which is not preferred. If the velocity of rotation of the spindle exceeds 20000 revolutions per minute, the noise and the heat generation of the spindle are increased, which is not desirable. Taking the velocity of rotation as described above into consideration, the optimum relative velocity between the magnetic recording medium and the magnetic head is 2 m/second to 30 m/second.